† Cyrtendoceratidae † Endoceratidae † Proterocameroceratidae † Yorkoceratidae Endocerida is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle.

Endocerids had a relatively small body chamber as well as a proportionally large siphuncle, which in some genera reached nearly half the shell diameter.

The chambers (camerae) of endocerids are always free of organic deposits, unlike orthoceratoid cephalopod orders such as the Orthocerida and Actinocerida.

The largest confirmed specimen, belonging to Endoceras giganteum, is 3 metres (9.8 ft) long as preserved, but is missing a substantial portion of its aboral end.

[5] Endocerids may have been the apex predators of the Ordovician, probably living close to the sea floor, and preying on trilobites, molluscs, brachiopods and other bottom-dwelling organisms.

They reached their greatest diversity during the Lower to Mid-Ordovician, but were already in decline by the middle of this period with most genera becoming extinct by the end of the Sandbian (late Ordovician), while some rare hangers on lasted into the Silurian.

Citing its diversity, Curt Teichert (1964) placed the Endocerida in its own subclass called the Endoceroidea or Endoceratoidea (which some Russian paleontologists ranked as a superorder instead.

Their generally orthoconic shell shape and dorsomyarian muscle scars are similar to the subclass Orthoceratoidea, which are ancestral to ammonoids (ammonites) and coleoids (squid, octopus, etc.).

[8] However, their nautilosiphonate connecting ring structure and lack of cameral deposits are more similar to living nautilus and their proposed ancestors, the subclass Multiceratoidea.